This invention reates to an electronic sphygmomanometer, and more particularly to a heartbeat referring type electronic sphygmomanometer.
It is evident that the measurement of blood pressure is necessary for regular medical treatment and health control. The measurement of blood pressure is required even in more so emergency medical treatments. When a conventional sphygmomanometer is used in a moving ambulance during emergency medical treatment, it is very much adversely affected by vibration and noise, and often operates erroneously. Therefore, the blood pressure cannot be measured accurately in many cases. A generally-used blood pressure measuring system includes a Korotkoff sound detecting device.
In the normal measurement sequence, a cuff placed over the brachial artery is first inflated to well beyond the systolic blood pressure thereby cutting off all blood flow. The cuff is then allowed to deflate slowly. When the cuff pressure reaches the systolic pressure, the pulsatile blood flow just begins and Korotkoff sounds will be detected. When the cuff pressure is further reduced to the diastolic blood pressure, the flow is continuous and no Korotkoff sounds are detected.
It is assumed that the pressures at which the first and last Korotkoff sounds are detected will correspond to the systolic and diastolic blood pressures respectively. An electronic sphygmomanometer is designed to electronically carry out the above-described operation, and is adapted to detect pressure variations (which will be hereinafter referred to as "pulse pressure variations") based on a pulsatile blood pressure variation within a brachial artery to use as a Korotkoff sound reference signal (which will be hereinafter referred to as "reference signal") for determining the existence of Korotkoff sounds. When such a sphygmomanometer is used in a moving vehicle, a cuff and rubber tubes are subjected to unavoidable vibration, and the vibration sounds enter not only a Korotkoff sound sensor but also the reference signal as noise, so that the sphygmomanometer is erroneously operated. A blood pressure measuring method utilizing as a reference signal a heartbeat synchronizing signal obtained by detecting cardioelectric potential has been developed for a considerably long period of time, but it is not practical due to a complicated step to detect cardioelectric potential.
The development of electronic circuits in recent years has brought an extra-small, inexpensive cardioelectric potential detector into existence, and made it possible to utilize heartbeats for sphygmomanometers conveniently. However, utilizing a heartbeat as a reference signal has some problems when the sphygmomanometer is used in a place in which noise or vibration occurs. Since heartbeats are measured constantly, reference signals are generated irrespective of the level of arm band pressure. Consequently, a sphygmomanometer is operated erroneously due to noise even when the cuff is in a level far away from the range of cuff pressures in which real Korotkoff sounds occur. Taking a logical product of heartbeats and a regular reference signal based on pulse pressure variations seems to be a good method of preventing this inconvenience. However, when a reference signal causes a false pulse to occur due to vibration, and if the time of occurrence of the false pulse agrees with that of a heartbeat synchronizing pulse, a plurality of pulses are generated per heartbeat synchronizing pulse if a method of taking such a logical product is employed. Therefore, heartbeats cannot be used as reliable reference signals.
In a conventional method of determining the presence of Korotkoff sounds, both the Korotkoff sound pulses and the Korotkoff sound reference signal are read into a CPU where they are compared by software. This method is advantageous in that its hardware is simple because the two signals are read directly into the CPU, but has a problem concerning loading on the software in that the relationship between the Korotkoff sound reference signal and the Korotkoff sound pulses must be examined continuously, or that a multiplexing interruption method must be used. The loading on the software not only causes problems in the arithmetical processing speed, but also the number of programs themselves is increased when the CPU is used for executing other operations as well as the blood pressure measurement.